In this problem, we study an important algorithm that uses the concepts seen this week. Consider a (target) distribution on Z. For all € Z we also have a (conditional) distribution qy with prob- ability mass function qy(r), and we apply the following algorithm: 1. Initialize zo € Z, and for i € {0, 1,..., n}, do: 2. Sample z from qr, (proposed value *) 3. Calculate P(x) = min (1, (1, n(x)qz (x₂) T(2i) (2) 4. Sample from Uniform([0,1]) DG Ja

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Problem 3 (Metropolis Hastings algorithm)
In this problem, we study an important algorithm that uses the concepts seen this week. Consider
a (target) distribution on Z. For all € Z we also have a (conditional) distribution qy with prob-
ability mass function qy(r), and we apply the following algorithm:
1. Initialize zo € Z, and for i = {0, 1,..., n}, do:
2. Sample à from qr, (proposed value à)
π (*) qz (x₂)
T(xi) qx, (x),
3. Calculate P(x) = min (1,
4. Sample from Uniform([0, 1])
=
5. If u P(x), accept new state: 4+1
6. If μ> P(x), reject new state:
+1=₁
Note that steps 3-6 simply means that the sampled state is accepted with probability P(x; 2) (and
rejected w.p. 1- P(x₁|x)).
a. Consider the algorithm output sequence (ro, 1,...,n) and assume that (x) > 0 if and only
if z ES, with zo ES. Show that for all k, xk € S.
We now consider a particular case where S = {1,2,3}, and qy is uniform over {y-1, y, y + 1}. We
also assume (1) ≤ (2) ≤ (3), and we study the Markov chain associated with x₂:
b. What are the transition probabilities from 1 (i.e. P₁ for 1 ≤ k ≤ 3)
c. What are the transition probabilities from 2?
d. Show that is stationary and that the process is reversible.
e. Does this result still hold when considering any conditional distribution qy?
Transcribed Image Text:Problem 3 (Metropolis Hastings algorithm) In this problem, we study an important algorithm that uses the concepts seen this week. Consider a (target) distribution on Z. For all € Z we also have a (conditional) distribution qy with prob- ability mass function qy(r), and we apply the following algorithm: 1. Initialize zo € Z, and for i = {0, 1,..., n}, do: 2. Sample à from qr, (proposed value à) π (*) qz (x₂) T(xi) qx, (x), 3. Calculate P(x) = min (1, 4. Sample from Uniform([0, 1]) = 5. If u P(x), accept new state: 4+1 6. If μ> P(x), reject new state: +1=₁ Note that steps 3-6 simply means that the sampled state is accepted with probability P(x; 2) (and rejected w.p. 1- P(x₁|x)). a. Consider the algorithm output sequence (ro, 1,...,n) and assume that (x) > 0 if and only if z ES, with zo ES. Show that for all k, xk € S. We now consider a particular case where S = {1,2,3}, and qy is uniform over {y-1, y, y + 1}. We also assume (1) ≤ (2) ≤ (3), and we study the Markov chain associated with x₂: b. What are the transition probabilities from 1 (i.e. P₁ for 1 ≤ k ≤ 3) c. What are the transition probabilities from 2? d. Show that is stationary and that the process is reversible. e. Does this result still hold when considering any conditional distribution qy?
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